Comparison of experimental and modelling studies for the microwave drying of Ironbark timber

Temperature profiles through boards during the microwave-assisted drying of Australian Ironbark timber have been investigated in this work in order to explore evidence for timber degrade via "charrirng" at internal temperatur es below those required for pyrolysis (Brooke et al., 1998). A previously p ublished model (Turner and Jolly, 1990a) describing one-dimensional microwa ve power absorption, based on the solution of Maxwell's equations, has been reviewed and significant limitations identified and overcome. Improvements included the use of a linear-mixing approach for the dielectric constants, the incorporation of temperature and moisture dependencies for these diele ctric properties and the inclusion of diffusion within an overall system mo del. A control-volume technique has been used for predicting both moisture and temperature distributions within the timber, with a second-order finite -difference method being used to solve Maxwell's equations. The final model showed evidence of "resonant" peaks in the temperature profile, but these were rapidly flattened by internal thermal conduction, Experimental work focussed mainly on the determination of the dynamic tempe rature profile through the cross-section of 250 mm x 240 mm x 25 mm thick p ieces of timber in a microwave oven with a continuous power output of 110 W . It proved possible to dry such timber without any internal "charring", an d as a result it can be concluded that no "thermal runaway" had occurred. S ignificant differences were found between the predicted and measured temper atures. A number of possible reasons for this have been investigated, indic ating that the ambient air conditions within the oven have a major impact o n the microwave-assisted drying of hardwood timber.